Vertex colors loose color data
Vertex Colors are held in 8-bit sRGB format rather than 'raw' (linear) RGB. Therefore, what actually appears to be 0.5 in sRGB space actually refers to a different 'raw' value - hence the need to convert. See How to convert image pixel values from 8bit to 32bit properly? - answered.. but why byte image buffer is stored with non-linear color values? for some related discussion and links.